[libcamera-devel] [PATCH v5 1/9] libcamera: base: utils: Provide defopt to simplify std::optional::value_or() usage

[Laurent Pinchart]

Hi Jacopo,

On Thu, Jul 28, 2022 at 08:47:05AM +0200, Jacopo Mondi wrote:
> Hi Laurent
> 
> On Thu, Jul 28, 2022 at 01:21:41AM +0300, Laurent Pinchart via libcamera-devel wrote:
> > The std::optional<T>::value_or(U &&default_value) function returns the
> > contained value if available, or \a default_value if the std::optional
> 
> Are the \a in the commit message intended ?

Oops :-) I'll drop it.

> > has no value. If the desired default value is a default-constructed T,
> > the obvious option is to call std::optional<T>::value_or(T{}). This
> > approach has two drawbacks:
> >
> > - The \a default_value T{} is constructed even if the std::optional
> >   instance has a value, which impacts efficiency.
> > - The T{} default constructor needs to be spelled out explicitly in the
> >   value_or() call, leading to long lines if the type is complex.
> >
> > Introduce a defopt variable that solves these issues by providing a
> > value that can be passed to std::optional<T>::value_or() and get
> > implicitly converted to a default-constructed T.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  include/libcamera/base/utils.h | 14 +++++++++
> >  src/libcamera/base/utils.cpp   | 21 +++++++++++++
> >  test/utils.cpp                 | 54 ++++++++++++++++++++++++++++++++++
> >  3 files changed, 89 insertions(+)
> >
> > diff --git a/include/libcamera/base/utils.h b/include/libcamera/base/utils.h
> > index cfff05836de7..889bb4a2270e 100644
> > --- a/include/libcamera/base/utils.h
> > +++ b/include/libcamera/base/utils.h
> > @@ -361,6 +361,20 @@ decltype(auto) abs_diff(const T &a, const T &b)
> >  		return a - b;
> >  }
> >
> > +namespace details {
> > +
> > +struct defopt_t {
> > +	template<typename T>
> > +	operator T() const
> > +	{
> > +		return T{};
> > +	}
> > +};
> > +
> > +} /* namespace details */
> > +
> > +constexpr details::defopt_t defopt;
> > +
> 
> Clever! this indeed explains how it works:
> 
> template< class U >
> constexpr T value_or( U&& default_value ) const&;
> 
> Equivalent to bool(*this) ? **this : static_cast<T>(std::forward<U>(default_value))
> 
> 
> Reviewed-by: Jacopo Mondi <jacopo at jmondi.org>
> 
> >  } /* namespace utils */
> >
> >  #ifndef __DOXYGEN__
> > diff --git a/src/libcamera/base/utils.cpp b/src/libcamera/base/utils.cpp
> > index 6a307940448e..9cd6cb197243 100644
> > --- a/src/libcamera/base/utils.cpp
> > +++ b/src/libcamera/base/utils.cpp
> > @@ -463,6 +463,27 @@ std::string toAscii(const std::string &str)
> >   * \a b
> >   */
> >
> > +/**
> > + * \var defopt
> > + * \brief Constant used with std::optional::value_or() to create a
> > + * default-constructed object
> > + *
> > + * The std::optional<T>::value_or(U &&default_value) function returns the
> > + * contained value if available, or \a default_value if the std::optional has no
> > + * value. If the desired default value is a default-constructed T, the obvious
> > + * option is to call std::optional<T>::value_or(T{}). This approach has two
> > + * drawbacks:
> > + *
> > + * * The \a default_value T{} is constructed even if the std::optional instance
> > + *   has a value, which impacts efficiency.
> > + * * The T{} default constructor needs to be spelled out explicitly in the
> > + *   value_or() call, leading to long lines if the type is complex.
> > + *
> > + * The defopt variable solves these issues by providing a value that can be
> > + * passed to std::optional<T>::value_or() and get implicitly converted to a
> > + * default-constructed T.
> > + */
> > +
> >  } /* namespace utils */
> >
> >  #ifndef __DOXYGEN__
> > diff --git a/test/utils.cpp b/test/utils.cpp
> > index d65467b5102c..129807a63ec6 100644
> > --- a/test/utils.cpp
> > +++ b/test/utils.cpp
> > @@ -7,6 +7,7 @@
> >
> >  #include <iostream>
> >  #include <map>
> > +#include <optional>
> >  #include <sstream>
> >  #include <string>
> >  #include <vector>
> > @@ -169,6 +170,55 @@ protected:
> >  		return TestPass;
> >  	}
> >
> > +	int testDefopt()
> > +	{
> > +		static bool defaultConstructed = false;
> > +
> > +		struct ValueType {
> > +			ValueType()
> > +				: value_(-1)
> > +			{
> > +				defaultConstructed = true;
> > +			}
> > +
> > +			ValueType(int value)
> > +				: value_(value)
> > +			{
> > +			}
> > +
> > +			int value_;
> > +		};
> > +
> > +		/*
> > +		 * Test that utils::defopt doesn't cause default-construction
> > +		 * of a ValueType instance when value_or(utils::defopt) is
> > +		 * called on a std::optional that has a value.
> > +		 */
> > +		std::optional<ValueType> opt = ValueType(0);
> > +		ValueType value = opt.value_or(utils::defopt);
> > +
> > +		if (defaultConstructed || value.value_ != 0) {
> > +			std::cerr << "utils::defopt didn't prevent default construction"
> > +				  << std::endl;
> > +			return TestFail;
> > +		}
> > +
> > +		/*
> > +		 * Then test that the ValueType is correctly default-constructed
> > +		 * when the std::optional has no value.
> > +		 */
> > +		opt = std::nullopt;
> > +		value = opt.value_or(utils::defopt);
> > +
> > +		if (!defaultConstructed || value.value_ != -1) {
> > +			std::cerr << "utils::defopt didn't cause default construction"
> > +				  << std::endl;
> > +			return TestFail;
> > +		}
> > +
> > +		return TestPass;
> > +	}
> > +
> >  	int run()
> >  	{
> >  		/* utils::hex() test. */
> > @@ -281,6 +331,10 @@ protected:
> >  		if (testDuration() != TestPass)
> >  			return TestFail;
> >
> > +		/* utils::defopt test. */
> > +		if (testDefopt() != TestPass)
> > +			return TestFail;
> > +
> >  		return TestPass;
> >  	}
> >  };

-- 
Regards,

Laurent Pinchart